Use of steel involving prolonged stressing at elevated temperatures



Aug. 17, 1965 Filed March 29, 1962 USE OF STEEL INVO H. W. R. THIER ELEVATED TEMPERATURES LVING PROLONGED STRESSING AT 2 Sheets-Sheet l X20 CIA/0 W V /2/ 10 CrMo 9/0 F/GJ United States Patent 3,201,232 USE OF STEEL INVULVING PRGLGNGED TRESS- ENG AT ELEVATED TEMPERATURES Hermann W. R. Thier, Grevenhroieh, Nordrhein, Germany, assignor to Gebr. Biihler Co. Aktiengesellschaft, Vienna, Austria Filed Mar. 29, 1962, Ser. No. 133568 Claims priority, applicationgGermany, Apr. 1, 1961,

06 '7 Claims. (Cl. 75-126) This invention relates to high-temperature steel having an optimum long-time creep resistance 031007000 and good working properties and more particularly to the use of steels containing 0.120.25% C, 75-85% Cr, l.52.5% M0, the balance being iron and the usual impurities including Si, Mn, S and P for applications in which said steel is stressed for long periods of time at temperatures exceeding 500 C., e.g. for superheaters of boilers. The steels of this invention may contain in addition one or more of the following elements V, W, Ti, B, Cb, Ta, N and Co in amounts which singly or together do not exceed 3%.

Chrome-moly steels are known which have low chromium and low molybdenum contents. These include 13 CrMo 44 and 10 CrMo 910 steels according to the German Standard Specification DIN 17,175. The 13 CrMo 44 steel is used only up to 560 C. and the 10 CrMo 910 steel is used only up to 575 C. because the rapid decrease of the long-time strength at elevated temperatures and the insufiicient resistance to scaling of these steels precludes their economical use at higher temperatures.

Very high-temperature, austenitic steels having a much higher creep resistance are available for higher temperatures. These include the X 8 CrNiNb 1613 steel, Material No. 49 61 of the Stahleisenliste, and the X 8 CrNiMoVNb 1613 steel, Material No. 4988. Owing to their high contents of alloying elements, the very high'temperature, austenitic steels are much more expensive than the hightemperature chrome-moly steels having a low contents of alloying elements and have some properties which have adversely affected their use, e.g., in the construction of boilers, in Germany. They are highly susceptible to stress crack corrosion, and have a much higher coeflicient of thermal expansion than ferritic steels. When they are welded together with ferritic steels, a prolonged stressing at elevated temperatures tends to cause diffusion of carbon from the ferritic to the austenitic steel. This results in decarburization and formation of a coarse gain structure in the ferritic steel whereby the creep resistance is reduced.

The long-time creep resistance values @3100010 of the two steel groups are plotted in FIG. 1 of the drawings, FIG. 2 is a similar graph relating to steels according to the invention.

The compositions of the steels referred to in FIGURES 1 and 2 are shown in the following table:

the very high-temperature, austenitic steels are unecononiical particularly owing to their high price.

The great expansion of steam power plants is one of the main reasons for an urgent requirement for steels capable of prolonged service at 550600 C., e.g., for the superheater of boilers, because the thermal efficiency of a boiler increases with the steam temperature attainable.

Attempts to close this gap have already been made. In Germany, the X 20 CrMoWV 121 steel, Material No. 4935, whichis also shown in FIG. 1, has been proposed. Similar complex alloy steels containing 7% or 8% chromium, approximately 3% molybdenum and approprimately 1% titanium and steels containing further alloying constituents such as columbium, aluminium, vanadium and tungsten have been investigated in Great Britain. All these steels must be subjected to a solution treatment and then to precipitation hardening at elevated temperature, by which the alloying elements initially dissolved in the steel should be precipitated in the form of carbides, nitrides and other intermetallic mixed phases such as Fe T i so as to retard the slip and how processes taking place under tensile stress at elevated temperatures. Unfortunately, the longtime creep resistance of these steels decreases strongly with an increase in temperature and at 600 C. does not substantially exceed the long-time creep resistance of 10 CrMo 910 Steel. This is shown by the example of X 20 CrMoWV 121 Steel, Material No. 4935, in FIG. 1.

Steels for short-time exposure (100-1000 hours) to high stresses, where the resistance to sealing is not significant, at temperatures of about 600 C., are disclosed in the U.S. Patent No. 2,835,571. In the specification of this patent, two steels having a lower molybdenum content are described as being much inferior for this service to steels having a higher molybdenum content and additions of manganese, titanium, vanadium, boron and nitrogen.

It may be quite correct that for short-time exposure (100-1000 hours) to high stresses (19-42 kg./sq. mm.) as are referred to in the specification of the above-mentioned U.S. patent, steels having high molybdenum contents of 2.7-3.4% have a higher creep resistance than steels containing less molybdenum. It is known, however, that tests carried out under high stress for a short time do not permit of conclusions as to the behavior during exposure to very long times, about 100,000 hours, under low stress. This is supported, e.g., by a publication of Dipl. Ing. A. von den Steinen of Deutsche Edelstahlwerke AG., Krefeld. In FIG. 1 of said publication it is shown that a specimen heat-treated to a higher strength (Condition 1) has up to about 3000 hours a higher creep resistance than a specimen heat-treated to a lower strength (Condition 2). These relations are in verted when the specimens are stressed for more than 3000 hours. The explanation given for this fact is that Specimen 2 has a more stable structure at the testing temperature than Specimen l. The Z steel referred to in the specification of the above-mentioned U.S. patent C Cr N1 M0 W V Nb OrNiMoVNb 1613.- rMo 44 It is distinctly apparent from FIG. 1 that there is a 70 gap particularly in the range of 550600 C. because in this range the low-alloy steels are no longer sufficient and as having the highest ultimate stress when stressed for hours has been tested, as stated in said specification, regarding theultimate stresses when stressed for 100 hours and 200 hours at 593 C., following various heat treatments. The long-time creep resistance values stated there are very high. These steels have not proved satisfactory, however, for long-time stressing and are not used, e.g., in the construction of boilers.

Because molybdenum may promote scaling under certain circumstances particularly in sulfur-containing gases as occur in boilers, steels to be stressed for long times should not have a very high molybdenum content. A high molybdenum content may also give rise to difii- =culties in the making and working of the steel.

Transactions of the American Society for Metals, contains in volume 37 a report on the high-temperature stability of Si-CrMo steels. In the book Molybdenum, Steels, Irons and Alloys contains reports on the ultimate stresses of steels containing l% chromium, 0.5 or 1% molybdenum and 1.21.5% silicon when stressed for 100,000 hours. The experiments underlying the present invention have shown that silicon has a highly detrimental influence on the long-time creep resistance.

For pure chrome-moly steels, recent German publications indicate that the creep resistance will even be at a minimum at medium chromium contents, as is stated e.g., in an article of 1. Class entitled Kennzeichnende Eigentumlichkeiten des warmfesten Cr-Etahles un Aussichten fur seine Einfuhrung in den Kesselbau, published in Mitteilungen der Vereinigung der Grobkesselbesitzer, No. 58, February 1959. In the same publication a minimum of the DVM creep limit is reported by H. Benneck and C. Bandel in Krupp-Forschungsberichte 1943 and a minimum of the reciprocal value of the overall creep according to E. W. Colbeck and I. R. Rait.

Hence, chrome-moly steels having medium chromium contents amounting to 7-9% Cr could hardly be used for applications requiring a prolonged stressing at elevated temperatures, e.g., in boilers.

As contrasted therewith, the invention teaches that a pure chrome-moly steel having a medium chromium content of 79% Cr and a molybdenum content of 1.02.5% Mo exists, which has satisfactory heat-treating properties and an optimum long-time creep resistance for pure chrome-moly steels. Molybdenum is known to have a strong tendency to form carbides and readily forms socalled special carbides. It is also known that molybdenum for-ms intermetallic compounds. Besides molybdenum increases the recrystallization temperature of iron by greatly impending the diffusion in the iron lattice. Owing to these properties the use of molybdenum in alloy steels improves the creep resistance.

It is also known that the fine division of the segregations in the solid solution increases the resistance to deformation at elevated temperatures under prolonged stressing. It is difficult, however, to achieve a very fine division of the carbides and to maintain such fine divisions for very long times.

In my opinion, increasing molybdenum contents and chromium contents resulting in a corresponding increase in the content of special carbides results in an increase of the creep resistance. This is in contrast with the pertinent technical literature. The creep resistance drops when the molybdenum and chromium contents are excessive because the carbides do not longer enter entirely into solution during the solution treatment and can no longer be completely brought into a state of fine division. In that case they are found in most cases in a coarsely coagulated state at the grain boundaries. As the solution treatment is carried out at increasing temperatures, larger amounts of carbides enter into solution so that the fineness of the segregation is promoted. The solubility, e.g., of molybdenum special carbides is much higher in the gamma solid solution than in the alpha solid solution. To enable the largest possible amount cfcarbides to enter into solution, followed by their segregation in a finely divided state, the invention teaches to select a steel composition which can be fully austenitized.

Thus, the upper limits for the chromium and molybdenum contents of such steel are set by the complete transformation from alpha to gamma. Because chromium and molybdenum in these alloys highly restrict the range in which the homogenous gamma phase exists, excessive contents of these elements will prevent a complete austenitization, a complete solution of the carbide and a refinement of the grain. The invention results in a particularly fine and uniform distribution of the segregations which increase the high-temperature strength when the transformation from gamma to alpha during the cooling from the solution treatment temperature is eliected as rapidly as possible or suddenly, as in the case of an eutectoid so that a pre-eutectoid segregation of coarse ferrite or coase carbides is avoided.

Based on these considerations, the following charges, inter alia, have been molten and investigated:

Steel No. C Si Mn Cr Mo V In all charges, P, S 0.020%. The steels were subjected to the following heat treatment:

Austenitizing at 1050 C. with air quenching Tempering at 700 C. for two hours.

The results of the creep tests at 600 C. are shown in FIG. 2. There was no fracture of the specimens of charges 243l, which specimens were stressed at 8 kg./ sq. min. These points, which are indicated in FIG. 2 by triangles for distinction from the broken specimens, must be considered displaced to longer times.

Steels 29 and 31 break after a much shorter time than the other steels. In accordance with the teaching of the invention this is due to the high silicon content. The element silicon highly restricts the gamma area without increasing the high-temperature strength. As contrasted therewith, FIG. 2 shows also the creep resistance values of 10 CrMo 910 Steel, 8 CrMo 36 10 Steel, X 20 CrMoWV 121 Steel, X 8 CrNiNb 1613 Steel, X 8 CrNiMoVNb Steel 1613. The creep strength investigation still in progress at the time of making the application indicate clearly that the steel according to the invention has a long-time creep resistance which is excellent for chromemoly steels.

The steel according to the present invention is nonscaling at temperatures up to and above 650 C. Owing to its high chromium content, its resistance to scaling is much higher than that of the low-alloy chrome-moly steels 13 CrMo 44 and 10 CrMo 910. Because V 0 melts at 665 C., vanadium-containing steels have in general a sufficient resistance to scaling only up to 600 C.

It has already been proved that steel according to the invention has satisfactory casting, piercing, rolling and tube-forming properties. The tubes may be hotand cold-worked. Due'to its simple composition, the making and working of the steel is very simple compared to complex alloy steels. Its economical use is also promoted by its relatively low price, which is due to the low contents of alloying constituents.

7 As has been mentioned in connection with the state of the art, a prolonged stressing of welded joints between the known 10 CrMo 910 and X 8 CrNiNb 1613 Steels results in a diffusion of carbon from ferritic to austenitic steel. If the steel according to the present invention is 5 used as a filler material in making the joint, the diifusion of carbon is highly impeded by the higher molybdenum content. Another advantage is the fact that the chromium content of the Weld is intermediate those of the ferritic and austenitic steels. For this reason the steel according to the invention as a filler material for the welded joints between austenitic and ferritic steels.

The welding properties can be improved by a reduction of the contents of Si, S, P of the steel according to this invention.

I claim:

1. A steel alloy having a high creep resistance for very long time intervals up to 100,000 hours and good corrosion resistance in uses wherein said steel is stressed for prolonged periods of time at temperatures exceeding 500 C., consisting essentially of 0.12-0.25 carbon, 7.58.5% chromium, 1.52.5% molybdenum and up to 3% of an element selected from the group consisting of vanadium, tungsten, titanium, boron, columbium, tantalum, nitrogen, cobalt, and mixtures thereof, the balance being iron and the usual impurities including silicon, manganese, sulfur and phosphorus, there being less than 0.2% sulfur, less than 0.02% phosphorus and less than 0.3% silicon in said alloy.

2. A steel alloy having a high creep resistance for very long time intervals up to 100,000 hours and good corrosion resistance in uses wherein said steel is stressed for prolonged periods of time at temperatures exceeding 575 C., consisting essentially of 0.120.25% carbon, 7.58.5% chromium, l.52.5% molybdenum and up to 3% of an element selected from the group consisting of vanadium, tungsten, titanium, boron, columbium, tantalum, nitrogen, cobalt, and mixtures thereof, the balance being iron and the usual impurities including silicon, manganese, sulfur and phosphorus, there being less than 0.02% sulfur, less than 0.02% phosphorus and less than 0.3% silicon in said alloy.

3. The steel alloy of claim 1, wherein said steel alloy is utilized in superheaters of boilers.

4, The steel alloy of claim 1, wherein said steel alloy is utilized as a filler material for Welded joints between ferritic and austenitic steels, said joints being subjected to stress for prolonged periods of time at temperatures exceeding 500 C.

5. A steel alloy having a high creep resistance for very long time intervals up to 100,000 hours and good corrosion resistance in uses wherein said steel alloy is stressed for prolonged periods of time at temperatures exceeding 500 C., consisting essentially of 0.12-0.25% carbon, 7.5-8.5% chromium, 1.5-2.5 molybdenum, the balance being iron and the usual impurities including silicon, manganese, sulfur and phosphorus, there being less than 0.02% sulfur, less than 0.02% phosphorus and less than 0.3% silicon in said alloy.

6. A steel alloy having a high creep resistance'for very long time intervals up to 100,000 hours and good corrosion resistance in uses wherein said steel alloy is stressed for prolonged periods of time at temperatures exceeding 500 C., consisting essentially of 0.12-0.25% carbon, 7.5-8.5% chromium, 1.52.5% molybdenum, and a carbide building element selected from the group consisting of vanadium, titanium, tungsten, boron, columbium, tantalum, and mixtures thereof in a quantity from a small but perceptible amount of 1%, the balance being iron and the usual impurities including silicon, manganese, sulfur and phosphorus, there being less than 0.02% sulfur, less than 0.02% phosphorus and less than 0.3% silicon in said alloy.

7. A steel consisting essentially of 0.15 to 0.25% carbon, 7.5 to 8.5% chromium, 1.5 to 2.5% molybdenum, less than 0.30% silicon, less than 0.02% phosphorus and 0.02% sulphur, the remainder being iron, as a material of optimum value properties for long time use at temperature above 575 C. for superheaters of steam boilers.

References Cited by the Examiner UNITED STATES PATENTS 2,123,144 7/38 NeWell l26 2,289,449 7/42 Nelson 75-126 2,835,571 5/5'8 Smith 75l26 FOREIGN PATENTS 702,555 1/54 Great Britain.

OTHER REFERENCES Ziegler et al.: Transaction, American Society For Metals, vol. 37, 1946, p. 362.

DAVID L. RECK, Primary Examiner.

MARCUS U. LYONS, RAY K. WINDHAM, Examiners. 

1. A STEEL ALLOY HAVING A HIGH CREEP RESISTANCE FOR VERY LONG TIME INTERVALS UP TO 100,000 HOURS AND GOOD CORROSION RESISTANCE IN USES WHEREIN SAID STEEL IS STRESSED FOR PROLONGED PERIODS OF TIME AT TEMPERATURES EXCEEDING 500* C., CONSISTING ESSENTIALLY OF 0.12-0.25% CARBON, 7.5-8.5% CHROMIUM, 1.5-2.5% MOLYBDENUM AND UP TO 3% OF AN ELEMENT SELECTED FROM THE GROUP CONSISTING OF VANADIUM, TUNGSTEN, TITANIUM, BORON, COLUMBIUM, TANTALUM, NITROGEN, COBALT, AND MIXTURES THEREOF, THE BALANCE BEING IRON AND THE USUAL IMPURITIES INCLUDING SILICON, MANGANESE, SULFUR AND PHOSPHORUS, THERE BEING LESS THAN 0.2% SULFUR, LESS THAN 0.02% PHOSPHORUS AND LESS THAN 0.3% SILICON IN SAID ALLOY. 